Bearing material

ABSTRACT

A plain bearing material, e.g. for a bearing bush or a thrust washer has a composition of 45 to 60 weight percent of semiaromatic polyamide, 15 to 20 weight percent of glass fibre, 10 to 20 weight percent of graphite, 10 to 20 weight percent of polytetrafluoroethylene and up to 5 weight percent of silicon oil.

This application is a 35 USC 371 of PCT/G.B.92/02339 filed Dec. 17,1992.

This invention is concerned with a plain bearing material suitable foruse e.g. as a bearing bush or as a thrust washer.

It is possible to use unfilled nylon 6.6 as a plain bearing material,e.g. for bearing bushes, but this material is not suitable forapplications requiring operation at high pressures and/or temperaturessince the wear of the material is unacceptable. The wear resistance ofthe material can be improved by mixing strengtheners and/or lubricantswith the nylon. For example, one bush that is on the market containsapproximately 30% by weight of glass fibres and approximately 15% byweight of polytetrafluoroethylene. Such bushes have an extended range ofoperation when compared to unfilled bushes but this range is stillconsiderably more limited than is desirable.

It is an object of the present invention to provide a plain bearingmaterial which has an extended range of operation in comparison withexisting materials.

The invention provides a plain bearing material having a compositioncomprising:

45 to 60 weight per cent of semi-aromatic polyamide,

15 to 25 weight per cent of glass fibre, 10 to 20 weight per cent ofgraphite, 10 to 20 weight per cent of polytetrafluoroethylene and up to5 weight per cent of silicon oil.

The semi-aromatic polyamide may be "Ultramid T" (Registered Trade Mark)obtainable from BASF. "Ultramid T" is a trade name for nylon 6/6 Tresins that contain both aromatic and aliphatic building blocks and areintended for injection moulding or extrusion. It is produced bypolycondensation of caprolactam, hexamethylene diamine, and terephthalicacid. By the term "silicon oil" is intended a polymethylsiloxane whichhas a high viscosity.

A plain bearing material in accordance with the invention is found toexhibit significantly better wear characteristics than existingmaterials and, accordingly, can be utilised over a greater range ofpressures and temperatures. The results obtained are considerablysuperior to those which might have been expected from the addition ofgraphite to the composition. Indeed as Table 2 of the attached Tablesshows, unfilled Ultramid T performed less well than unfilled Nylon 6.6,but, contrary to expectations, a material according to the invention wassuperior to a material containing nylon 6.6 instead of Ultramid T.

Preferably, in a bearing material in accordance with the invention, thecomposition comprises 1 to 3 weight per cent of the silicon oil. Thegraphite and the polytetrafluoroethylene may be present in approximatelyequal quantities, e.g. 15% of graphite and 13% ofpolytetrafluoroethylene.

The material of the invention may be made into a bearing bush byinjection moulding, or other means. Such a bearing bush may have aflange which may provide a thrust surface. The material mayalternatively be made into a thrust washer.

There now follows a detailed description of an example of a materialwhich is illustrative of the invention.

The material of the illustrative example was compounded to formgranules. The material had a composition comprising 50% by weight ofUltramid T (Registered Trade Mark) semi-aromatic polyamide, 20 weightper cent of glass fibre, 15 weight per cent of graphite, 13 weight percent of polytetrafluoroethylene, and 2 weight per cent of silicon oil.The glass fibre was E-glass obtainable from Owens Corning (grade R23D).The graphite was synthetic graphite obtained from Lonza (grade K5 5/75,ie substantially no particles below 5 mm in diameter or above 75 mm indiameter). The polytetrafluoroethylene was "Fluon" (Registered TradeMark) L 169 obtained from ICI. The silicon oil was obtained from DowCorning (grade DC 200/30000).

The granules were injection moulded to form cylindrical bearing bushes20 mm in diameter, 15 mm in length and 1.5 mm in wall thickness. Thesebushes were then subjected to wear rate tests as were bushes of the samedimensions made from unfilled Nylon 6.6, Nylon 6.6 filled with 20% glassfibre, 15% graphite, 13% polytetrafluoroethylene and 2% silicon oil, i.ethe same filling materials in the same percentages as in the example. Acommercially available bush having the same dimensions and a compositionof Nylon 6.6 filled with 30% fibre and 15% polytetrafluoroethylene wasalso tested. The results of the tests are summarised in the attachedTables.

Table 1 shows a comparison of wear rates (expressed in micrometres perhundred hours) at various PV (pressure times velocity) values (expressedin Mega Pascal metres per second) the test speed being 750 RPM. It canbe seen that, at PV values of 0.1, 0.21 and 0.35, the compositionaccording to the invention consistently out performed the commerciallyavailable product with the improved performance increasing at higher PVvalues.

Table 2 shows a comparison at a PV value of 0.1 of the wear rates offilled and unfilled Ultramid T and Nylon 6.6, the filling being the samein both cases and being 20% glass fibre, 15% graphite, 13%polytetrafluoroethylene and 2% silicon oil. The unfilled Nylon 6.6 outperformed the unfilled Ultramid T but, contrary to expectation, thefilled Ultramid T out performed the filled Nylon 6.6 very significantly.Table 2 shows that unfilled Nylon 6.6 has a very good wear rate butunfilled Nylon 6.6 cannot be used at higher PV values. Ultramid T filledaccording to the invention can be used at higher PV values and has awear rate significantly better than filled Nylon 6.6.

Table 3 shows wear rate results at a PV of 0.21 from a different type oftesting machine to that used for Tables 1 and 2. Table 3 confirms thatthe composition according to the example significantly out performsfilled Nylon 6.6.

                  TABLE 1                                                         ______________________________________                                        MATERIAL        P.V.   WEAR RATE                                              ______________________________________                                        Ultramid T      0.1    25                                                     20% Glass Fibre 0.21   58                                                     15% Graphite    0.35   165                                                    13% PTFE                                                                      2% Silicon Oil                                                                Nylon 6.6       0.1    44                                                     30% Glass Fibre 0.21   118                                                    15% PTFE        0.35   559                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        MATERIAL      NYLON 6.6  UTRAMID T                                            ______________________________________                                        Unfilled      20         88                                                   Filled        40         25                                                   ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        MATERIAL         PV     WEAR RATE                                             ______________________________________                                        Ultramid T       0.21    77                                                   20% Glass Fibre                                                               15% Graphite                                                                  13% PTFE                                                                      2% Silicon Oil                                                                Nylon 6.6        0.21   170                                                   30% Glass Fibre                                                               15% PTFE                                                                      Nylon 6.6        0.21   153                                                   20% Glass Fibre                                                               15% Graphite                                                                  13% PTFE                                                                      2% Silicon Oil                                                                ______________________________________                                    

I claim:
 1. A plain bearing material having a composition comprising: 45to 60 weight per cent of semi-aromatic polyamide, 15 to 25 weight percent of glass fibre, 10 to 20 weight per cent of graphite, 10 to 20weight percent of polytetrafluoroethylene, and up to 5 weight percent ofsilicon oil.
 2. A bearing material according to claim 1, characterisedin that the semi-aromatic polyamide is a co-polymer of caprolactam,hexamethylene, diamine and terephthalic acid.
 3. A bearing materialaccording to claim 1, characterised in that the composition comprises1-3 weight percent of silicon oil.
 4. A bearing bush made of a materialaccording to either one of claims 1 and
 2. 5. A bearing bush accordingto claim 4, characterised in that the bush has at least one flangethereon to act as a thrust surface.
 6. A thrust washer made of amaterial according to either one of claims 1 and 2.